1. Field of the Invention
The present invention relates to a fuel injection control device used in an internal combustion engine for a vehicle to drive an electromagnetic valve for fuel injection at high speed in accordance with a pressure of fuel discharged from high-pressure fuel pump.
2. Description of the Related Art
A fuel injection device for supplying fuel to an internal combustion engine for a vehicle, which is composed of a valve opening signal generating unit, a power supply control unit, a fuel pump, a main power supply, an auxiliary power supply, and the like, has been in widespread use. The valve opening signal generating unit calculates an injection amount of fuel to be supplied based on signals from a group of sensors for detecting operation states of the internal combustion engine, determines a valve opening timing and a valve closing time period for fuel injection valves, and outputs a valve opening signal. The power supply control unit applies a high voltage to an electromagnetic coil for driving the fuel injection valves in accordance with the valve opening signal outputted from the valve opening signal generating unit, and holds the fuel injection valves open through a small current after the fuel injection valves have been driven to be opened quickly. The fuel pump supplies high-pressure fuel having a variable pressure from the fuel injection valves to the internal combustion engine. The main power supply supplies a power to the group of the sensors, the power supply control unit, and the like. The auxiliary power supply raises the voltage of the main power supply, and supplies a current to the electromagnetic coil for driving the fuel injection valves when they are opened.
For example, there is a fuel injection device as disclosed in JP 2000-303882 A. This fuel injection device is equipped with a fuel pressure regulator for adjusting a fuel pressure of injected fuel, an injector driving unit for driving valve bodies of injectors to be opened and closed by energizing an electromagnetic coil for the injectors, and an injection control unit for controlling the fuel pressure regulator and the injector driving unit in accordance with a rotational speed, an intake air amount, and a fuel pressure of the internal combustion engine. The injection control unit has an injection timer for setting a time period for exciting the electromagnetic coil, an overexcitation timer for supplying an overexcitation current in an initial stage of energization, and an overexcitation period control portion for variably setting the overexcitation timer in accordance with a fuel pressure. An overexcitation period is variably set to be longer as the fuel pressure rises.
In order to enlarge the linearity of an injection amount with respect to an injection pulse time period of the injectors, it is necessary to reduce the operating time period in turning the valve bodies of the injectors ON and OFF and accurately control them to be opened and closed. The time period in which the valve bodies of the injectors are ON can be made shorter as the amount of the current increases, and the time period in which the valve bodies of the injectors are OFF can be made shorter as the amount of the current decreases. In accordance with this principle, when opening the valve bodies of the injectors, the injector driving unit supplies a predetermined overexciting current to the electromagnetic coil for driving the valve bodies of the injectors with a steep gradient, and keeps supplying the overexciting current until the operation of turning the valve bodies ON is substantially completed. On the other hand, the electromagnetic suction force of the electromagnetic coil increases after the operation of turning the valve bodies ON has been completed. Therefore, in order to maintain the valve bodies of the injectors fully open, the electromagnetic coil may be supplied with a minimum required holding current that is smaller than the overexcitation current.
However, if the time period for supplying the overexcitation current is set relatively long, the injectors are driven with a large current when being turned OFF in the case where the injection time period is short. As a result, the time period in which the valves are OFF is long. Accordingly, it is desirable to set the time period for supplying the overexcitation current as short as possible. The opening time period of the injector valves is longer as the fuel pressure rises, because of the structure of the valves. Thus, an overexcitation time period is changed in accordance with the fuel pressure, and a minimum required overexcitation time period is set to open the valves of the injectors. In consequence, optimum control is performed.
In the technology disclosed in JP 2000-303882 A, the overexcitation time period is changed in accordance with the fuel pressure, and the operating time period for turning the valve bodies of the injectors ON and OFF is shortened to accurately control the valve bodies to be opened and closed. However, the pressure of fuel supplied to the fuel injection valves cannot be accurately recognized when there is a malfunction in a fuel pressure sensor for setting the overexcitation time period. Therefore, the overexcitation time period is set inappropriately. In the worst case, the inappropriate overexcitation time period leads to a problem in that the fuel injection valves are not opened and the fuel supply is impossible.